Toward fabric-based flexible microfluidic devices: pointed surface modification for pH sensitive liquid transport

Advances in textile-based microfluidics for biomolecule sensing

Textile-based microfluidic biosensors represent an innovative fusion of various multidisciplinary fields, including bioelectronics, material sciences, and microfluidics. Their potential in biomedicine is significant as they leverage textiles to achieve high demands of biocompatibility with the human body and conform to the irregular surfaces of the body. In the field of microfluidics, fabric coated with hydrophobic materials serves as channels through which liquids are transferred in precise amounts to the sensing element, which in this case is a biosensor. This paper presents a condensed overview of the current developments in textile-based microfluidics and biosensors in biomedical applications over the past 20 years (2005-2024). A literature search was performed using the Scopus database. The fabrication techniques and materials used are discussed in this paper, as these will be key in various modifications and advancements in textile-based microfluidics. Furthermore, we also address the gaps in the application of textile-based microfluidic analytical devices in biomedicine and discuss the potential solutions. Advances in textile-based microfluidics are enabled by various printing and fabric manufacturing techniques, such as screen printing, embroidery, and weaving. Integration of these devices into everyday clothing holds promise for future vital sign monitoring, such as glucose, albumin, lactate, and ion levels, as well as early detection of hereditary diseases through gene detection. Although most testing currently takes place in a laboratory or controlled environment, this field is rapidly evolving and pushing the boundaries of biomedicine, improving the quality of human life.

A cost-effective and facile technique for realizing fabric based microfluidic channels using beeswax and PVC stencils

Microfluidic channels fabricated over fabrics or papers have the potential to find substantial application in the next generation of wearable healthcare monitoring systems. The present work focuses on the fabrication procedures that can be used to obtain practically realizable fabric-based microfluidic channels (μFADs) utilizing patterning masks and wax, unlike conventional printing techniques. In this study, comparative analysis was used to differentiate channels obtained using different masking tools for channel patterning as well as different wax materials as hydrophobic barriers. Drawbacks of the conventional tape and candle wax technique were noted and a novel approach was used to create microfluidic channels through a facile and simple masking technique using PVC clear sheets as channel stencils and beeswax as the channel barriers. The resulting fabric based microfluidic channels with varying widths as well as complex microchannel, microwell, and micromixer designs were investigated and a minimum channel width resolution of 500 μm was successfully obtained over cotton based fabrics. Thereafter, the PVC clear sheet-beeswax based microwells were successfully tested to confine various organic and inorganic samples indicating vivid applicability of the technique. Finally, the microwells were used to make a simple and facile colorimetric assay for glucose detection and demonstrated effective detection of glucose levels from 10 mM to 50 mM with significant color variation using potassium iodide as the coloring agent. The above findings clearly suggest the potential of this alternative technique for making low-cost and practically realizable fabric based diagnostic devices (μFADs) in contrast to the other approaches that are currently in use.

Materials and Structural Designs toward Motion Artifact-Free Bioelectronics

Bioelectronics encompassing electronic components and circuits for accessing human information play a vital role in real-time and continuous monitoring of biophysiological signals of electrophysiology, mechanical physiology, and electrochemical physiology. However, mechanical noise, particularly motion artifacts, poses a significant challenge in accurately detecting and analyzing target signals. While software-based "postprocessing" methods and signal filtering techniques have been widely employed, challenges such as signal distortion, major requirement of accurate models for classification, power consumption, and data delay inevitably persist. This review presents an overview of noise reduction strategies in bioelectronics, focusing on reducing motion artifacts and improving the signal-to-noise ratio through hardware-based approaches such as "preprocessing". One of the main stress-avoiding strategies is reducing elastic mechanical energies applied to bioelectronics to prevent stress-induced motion artifacts. Various approaches including strain-compliance, strain-resistance, and stress-damping techniques using unique materials and structures have been explored. Future research should optimize materials and structure designs, establish stable processes and measurement methods, and develop techniques for selectively separating and processing overlapping noises. Ultimately, these advancements will contribute to the development of more reliable and effective bioelectronics for healthcare monitoring and diagnostics.

A Step Forward for Smart Clothes─Fabric-Based Microfluidic Sensors for Wearable Health Monitoring

We report the first demonstration of fabric-based microfluidics for wearable sensing. A new technology to develop microfluidics on fabrics, as a part of an undergarment, is described here. Compared to conventional microfluidics from polydimethylsiloxane, fabric-based microfluidics are simple to make, robust, and suitable for efficient sweat delivery. Specifically, acrylonitrile butadiene styrene (ABS) films with precut microfluidic patterns were infused through fabrics to form hydrophobic areas in a specially controlled sandwich structure. Experimental tests and simulations confirmed the sweat delivery efficiency of the microfluidics. Electrodes were screen-printed onto the fabric-based microfluidic. A novel wearable potentiometer based on Arduino was also developed as the transducer and signal readouts, which was low-cost, standardized, open-source, and capable of wireless data transfer. We applied the sensor system as a standalone or as a module of a T-shirt to quantify [Ca²⁺] in a wearer's sweat, with physiological and accurate results generated. Overall, this work represents a critical step in turning regular undergarments into biochemically smart platforms for health monitoring, which will broadly benefit human healthcare.

Sol-Gel Assisted Immobilization of Alizarin Red S on Polyester Fabrics for Developing Stimuli-Responsive Wearable Sensors

In the field of stimuli-responsive materials, introducing a pH-sensitive dyestuff onto textile fabrics is a promising approach for the development of wearable sensors. In this paper, the alizarin red S dyestuff bonded with a sol-gel precursor, namely trimethoxy-[3-(oxiran-2-ylmethoxy)propyl]silane, was used to functionalize polyethylene terephthalate fabrics, a semi-crystalline thermoplastic polyester largely used in the healthcare sector mainly due to its advantages, including mechanical strength, biocompatibility and resistance against abrasion and chemicals. The obtained hybrid halochromic silane-based coating on polyester fabrics was investigated with several chemical characterization techniques. Fourier transform infrared spectroscopy and X-ray Photoelectron Spectroscopy confirmed the immobilization of the dyestuff-based silane matrix onto polyethylene terephthalate samples through self-condensation of hydrolyzed silanols under the curing process. The reversibility and repeatability of pH-sensing properties of treated polyester fabrics in the pH range 2.0-8.0 were confirmed with diffuse reflectance and CIELAB color space characterizations. Polyester fabric functionalized with halochromic silane-based coating shows the durability of halochromic properties conversely to fabric treated with plain alizarin red S, thus highlighting the potentiality of the sol-gel approach in developing durable halochromic coating on synthetic substrates. The developed wearable pH-meter device could find applications as a non-invasive pH sensor for wellness and healthcare fields.

Wireless bipolar electrode-based textile electrofluidics: towards novel micro-total-analysis systems

Point of care testing using micro-total-analysis systems (μTAS) is critical to emergent healthcare devices with rapid and robust responses. However, two major barriers to the success of this approach are the prohibitive cost of microchip fabrication and poor sensitivity due to small sample volumes in a microfluidic format. Here, we aimed to replace the complex microchip format with a low-cost textile substrate with inherently built microchannels using the fibers' spaces. Secondly, by integrating this textile-based microfluidics with electrophoresis and wireless bipolar electrochemistry, we can significantly improve solute detection by focusing and concentrating the analytes of interest. Herein, we demonstrated that an in situ metal electrode simply inserted inside the textile-based electrophoretic system can act as a wireless bipolar electrode (BPE) that generates localized electric field and pH gradients adjacent to the BPE and extended along the length of the textile construct. As a result, charged analytes were not only separated electrophoretically but also focused where their electrophoretic migration and counter flow (EOF) balances due to redox reactions proceeding at the BPE edges. The developed wireless redox focusing technique on textile constructs was shown to achieve a 242-fold enrichment of anionically charged solute over an extended time of 3000 s. These findings suggest a simple route that achieves separation and analyte focusing on low-cost surface-accessible inverted substrates, which is far simpler than the more complex ITP on conventional closed and inaccessible capillary channels.

Knitting Thread Devices: Detecting Candida albicans Using Napkins and Tampons

Reproducible and in situ microbial detection, particularly of microbes significant in urinary tract infections (UTIs) such as Candida albicans, provides a unique opportunity to bring equity in the healthcare outcomes of disenfranchised groups like women in low-resource settings. Here, we demonstrate a system to potentially detect vulvovaginal candidiasis by leveraging the properties of multifilament cotton threads in the form of microfluidic-thread-based analytical devices (μTADs) to develop a frugal microbial identification assay. A facile mercerization method using heptane wash to boost reagent absorption and penetration is also performed and is shown to be robust compared to other existing conventional mercerization methods. Furthermore, the twisted mercerized fibers are drop-cast with media consisting of l-proline β-naphthylamide, which undergoes hydrolysis by the enzyme l-proline aminopeptidase secreted by C. albicans, hence signaling the presence of the pathogen via simple color change with a limit of detection of 0.58 × 106 cfu/mL. The flexible and easily disposable thread-based detection device when integrated with menstrual hygiene products showed a detection time of 10 min using spiked vaginal discharge. The developed method boasts a long shelf life and high stability, making it a discreet detection device for testing, which provides new vistas for self-testing multiple diseases that are considered taboo in certain societies.

Thread integrated smart-phone imaging facilitates early turning point colorimetric assay for microbes

This study employs a commercial multifilament cotton thread as a low-cost microbial identification assay integrated with smartphone-based imaging for high throughput and rapid detection of pathogens. The thread device with inter-twined fibers was drop-cast with test media and a pH indicator. The target pathogens scavenge the media components with different sugars and release acidic by-products, which in turn act as markers for pH-based color change. The developed thread-based proof-of-concept was demonstrated for the visual color detection (red to yellow) of Candida albicans (≈16 hours) and Escherichia coli (≈5 hours). Besides that, using a smart-phone to capture images of the thread-based colorimetric assay facilitates early detection of turning point of the pH-based color change and further reduces the detection time of pathogens viz. Candida albicans (≈10 hours) and Escherichia coli (≈1.5 hours). The reported thread and smartphone integrated image analysis works towards identifying the turning point of the colorimetric change rather than the end-point analysis. Using this approach, the interpretation time can be significantly reduced compared to the existing conventional microbial methods (≈24 hours). The thread-based colorimetric microbial assay represents a ready-to-use, low-cost and straightforward technology with applicability in resource-constrained environments, surpassing the need for frequent fresh media preparation, expensive instrumentation, complex fabrication techniques and expert intervention. The proposed method possesses high scalability and reproducibility, which can be further extended to bio(chemical) assays.

Microfluidic cloth-based analytical devices: Emerging technologies and applications

Cloth (or fabric) is an omnipresent material that has various applications in everyday life, and has become one of the things people are most familiar with. It has some attractive properties such as low cost, ability to transport fluid by capillary force, high tensile strength and durability, good wet strength, and great biocompatibility and biodegradability. Hence, cloth is an ideal material for the development of economical and user-friendly diagnostic devices for many applications including food detection, environmental monitoring, disease diagnosis and public health. Microfluidic cloth-based analytical devices (μCADs) (or microfluidic fabric-based analytical devices (μFADs)) first emerged in 2011 as a low-cost alternative to conventional laboratory testing, with the goal of improving point of care testing and disease screening in the developing world. In this review, we examine the advances in the development of μCADs from 2011 to 2020, especially highlighting emerging technologies and applications related to the μCADs. First, different fabrication methods for μCADs are introduced and compared. Second, a series of cloth-based microfluidic functional components are discussed, including microvalves, fluid velocity control elements, micromixers, and microfilters. Then, electroanalytical μCADs are described, especially focusing on the use of cloth-based electrodes. Next, various detection methods for μCADs, together with their corresponding applications, are compared and categorized. In addition, the current development of wearable μCADs is also demonstrated. Finally, the future outlook and trends in this field are discussed.

Life-Saving Threads: Advances in Textile-Based Analytical Devices

Novel approaches that incorporate electrofluidic and microfluidic technologies are reviewed to illustrate the translation of traditional enclosed structures into open and accessible textile based platforms. Through the utilization of on-fiber and on-textile microfluidics, it is possible to invert the typical enclosed capillary column or microfluidic "chip" platform, to achieve surface accessible efficient separations and fluid handling, while maintaining a microfluidic environment. The open fiber/textile based fluidics approach immediately provides new possibilities to interrogate, manipulate, redirect, extract, characterize, and quantify solutes and target species at any point in time during such processes as on-fiber electrodriven separations. This approach is revolutionary in its simplicity and provides many potential advantages not otherwise afforded by the more traditional enclosed platforms.

Low-Cost Chemical-Responsive Adhesive Sensing Chips

Chemical-responsive adhesive sensing chip is a new low-cost analytical platform that uses adhesive tape loaded with indicator reagents to detect or quantify the target analytes by directly sticking the tape to the samples of interest. The chemical-responsive adhesive sensing chips can be used with paper to analyze aqueous samples; they can also be used to detect and quantify solid, particulate, and powder analytes. The colorimetric indicators become immediately visible as the contact between the functionalized adhesives and target samples is made. The chemical-responsive adhesive sensing chip expands the capability of paper-based analytical devices to analyze solid, particulate, or powder materials via one-step operation. It is also a simpler alternative way, to the covalent chemical modification of paper, to eliminate indicator leaching from the dipstick-style paper sensors. Chemical-responsive adhesive chips can display analytical results in the form of colorimetric dot patterns, symbols, and texts, enabling clear understanding of assay results by even nonprofessional users. In this work, we demonstrate the analyses of heavy metal salts in silica powder matrix, heavy metal ions in water, and bovine serum albumin in an aqueous solution. The detection is one-step, specific, sensitive, and easy-to-operate.

A low-cost, ultraflexible cloth-based microfluidic device for wireless electrochemiluminescence application

The rising need for low-cost diagnostic devices has led to the search for inexpensive matrices that allow performing alternative analytical assays. Cloth is a viable material for the development of analytical devices due to its low material and manufacture costs, ability to wick assay fluids by capillary forces, and potential for patterning multiplexed channel geometries. In this paper, we describe the construction of low-cost, ultraflexible microfluidic cloth-based analytical devices (μCADs) for wireless electrochemiluminescence based on closed bipolar electrodes (C-WL-ECL), employing extremely cheap materials and a manufacturing process. The C-WL-ECL μCADs are built with wax-screen-printed cloth channels and carbon ink screen-printed electrodes, and the estimated cost per device is only $0.015. To demonstrate the performance of C-WL-ECL μCADs, the two most commonly used ECL systems - tris(2,2'-bipyridyl)ruthenium(ii)/tri-n-propylamine (Ru(bpy)3(2+)/TPA) and 3-aminophthalhydrazide/hydrogen peroxide (luminol/H2O2) - are applied. Under optimized conditions, the C-WL-ECL method has successfully fulfilled the quantitative determination of TPA with a detection limit of 0.085 mM. In addition, on the bent μCADs (bending angle (θ) = 180°), the luminol/H2O2-based ECL system can detect H2O2 as low as 0.024 mM. Based on such an ECL system, the bent μCADs are further used for determination of glucose in a phosphate buffer solution (PBS), with the detection limit of 0.195 mM. Finally, the applicability and validity, anti-interference ability, and storage stability of the C-WL-ECL μCADs are investigated. The results indicate that the proposed device has shown potential to extend the use of microfluidic analytical devices, due to its simplicity, low cost, ultraflexibility, and acceptable analytical performance.

Chemiluminescence detection for microfluidic cloth-based analytical devices (μCADs)

In this work, we report the first demonstration of chemiluminescence (CL) detection for microfluidic cloth-based analytical devices (μCADs). Wax screen-printing is used to make cloth channels or chambers, and enzyme-catalyzed CL reactions are imaged using an inexpensive charge coupled device (CCD). We first evaluate the relationship between the wicking rate and the length/width of cloth channel. For our device, the channel length and width between the loading and detection chambers are optimized to be 10mm and 3mm. Thus, the detection procedure can be accomplished in about 15s on a cloth-based device (15 × 30 mm(2)) by using 25-μL sample spotted on it. Next, several parameters affecting cloth-based CL intensity are studied, including exposure time, pH, and concentrations of luminol and enzyme. Under optimal conditions, a linear relationship is obtained between CL intensity and hydrogen peroxide (H2O2) concentrations in the range of 0.5-5mM with a detection limit of 0.46 mM. Finally, the utility of cloth-based CL is demonstrated for determination of H2O2 residues in meat samples. On our device, the chicken meat soaked for 6h with 3% H2O2 can be detected. Moreover, the supernatant of grinded meat sample can be directly applied, without need for other treatments. We believe that μCADs with CL detection could provide a new platform of rapid and low-cost assays for use in areas such as food detection and environmental monitoring.

Low-cost, high-throughput fabrication of cloth-based microfluidic devices using a photolithographical patterning technique

In this work, we first report a facile, low-cost and high-throughput method for photolithographical fabrication of microfluidic cloth-based analytical devices (μCADs) by simply using a cotton cloth as a substrate material and employing an inexpensive hydrophobic photoresist laboratory-formulated from commercially available reagents, which allows patterning of reproducible hydrophilic-hydrophobic features in the cloth with well-defined and uniform boundaries. Firstly, we evaluated the wicking properties of cotton cloths by testing the wicking rate in the cloth channel, in combination with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses. It is demonstrated that the wicking properties of the cloth microfluidic channel can be improved by soaking the cloth substrate in 20 wt% NaOH solution and by washing the cloth-based microfluidic patterns with 3 wt% SDS solution. Next, we studied the minimum dimensions achievable for the width of the hydrophobic barriers and hydrophilic channels. The results indicate that the smallest width for a desired hydrophobic barrier is designed to be 100 μm and that for a desired hydrophilic channel is designed to be 500 μm. Finally, the high-throughput μCADs prepared using the developed fabrication technique were demonstrated for colorimetric assays of glucose and protein in artificial urine samples. It has been shown that the photolithographically patterned μCADs have potential for a simple, quantitative colorimetric urine test. The combination of cheap cloth and inexpensive high-throughput photolithography enables the development of new types of low-cost cloth-based microfluidic devices, such as "microzone plates" and "gate arrays", which provide new methods to perform biochemical assays or control fluid flow.

A highly flexible and compact magnetoresistive analytic device

A grand vision of realization of smart and compact multifunctional microfluidic devices for wearable health monitoring, environment sensing and point-of-care tests emerged with the fast development of flexible electronics. As a vital component towards this vision, magnetic functionality in flexible fluidics is still missing although demanded by the broad utility of magnetic nanoparticles in medicine and biology. Here, we demonstrate the first flexible microfluidic analytic device with integrated high-performance giant magnetoresistive (GMR) sensors. This device can be bent to a radius of 2 mm while still retaining its full performance. Various dimensions of magnetic emulsion droplets can be probed with high precision using a limit of detection of 0.5 pl, providing broad applicability in high-throughput droplet screening, flow cytometry and drug development. The flexible feature of this analytic device holds great promise in the realization of wearable, implantable multifunctional platforms for biomedical, pharmaceutical and chemical applications.

Meniscus formation in a capillary and the role of contact line friction

We studied spontaneous formation of an internal meniscus by dipping glass capillaries of 25 μm to 350 μm radii into low volatile hexadecane and tributyl phosphate. X-ray phase contrast and high speed optical microscopy imaging were employed. We showed that the meniscus completes its formation when the liquid column is still shorter than the capillary radius. After that, the meniscus travels about ten capillary radii at a constant velocity. We demonstrated that the experimental observations can be explained by introducing a friction force linearly proportional to the meniscus velocity with a friction coefficient depending on the air/liquid/solid triplet. It was demonstrated that the friction coefficient does not depend on the capillary radius. Numerical solution of the force balance equation revealed four different uptake regimes that can be specified in a phase portrait. This phase portrait was found to be in good agreement with the experimental results and can be used as a guide for the design of thin porous absorbers.

Exploration of microfluidic devices based on multi-filament threads and textiles: A review

In this paper, we review the recent progress in the development of low-cost microfluidic devices based on multifilament threads and textiles for semi-quantitative diagnostic and environmental assays. Hydrophilic multifilament threads are capable of transporting aqueous and non-aqueous fluids via capillary action and possess desirable properties for building fluid transport pathways in microfluidic devices. Thread can be sewn onto various support materials to form fluid transport channels without the need for the patterned hydrophobic barriers essential for paper-based microfluidic devices. Thread can also be used to manufacture fabrics which can be patterned to achieve suitable hydrophilic-hydrophobic contrast, creating hydrophilic channels which allow the control of fluids flow. Furthermore, well established textile patterning methods and combination of hydrophilic and hydrophobic threads can be applied to fabricate low-cost microfluidic devices that meet the low-cost and low-volume requirements. In this paper, we review the current limitations and shortcomings of multifilament thread and textile-based microfluidics, and the research efforts to date on the development of fluid flow control concepts and fabrication methods. We also present a summary of different methods for modelling the fluid capillary flow in microfluidic thread and textile-based systems. Finally, we summarized the published works of thread surface treatment methods and the potential of combining multifilament thread with other materials to construct devices with greater functionality. We believe these will be important research focuses of thread- and textile-based microfluidics in future.

Characterization of permeability of electrospun yarns

We developed a novel technique enabling determination of the permeability of electrospun yarns composed of hundreds of fibers. Analyzing the wicking kinetics in a yarn-in-a-tube composite conduit, it was found that the kinetic is very specific. The liquid was pulled by the capillary pressure associated with the meniscus in the tube while the main resistance comes from the yarn. Therefore, one can separate the yarn permeability from the capillary pressure, which cannot be done in wicking experiments with single yarns. A surface tensiometer (Cahn) was employed to collect the data on wicking kinetics of hexadecane into the yarn-in-a-tube conduits. Yarns from different polymers and blends were electrospun and characterized using the proposed protocol. We showed that the permeability of electrospun yarns can be varied in a broad range from 10(-14) m(2) to 10(-12) m(2) by changing the fiber diameter and packing density. These results offer new applications of electrospun yarns as flexible micro- and nanofluidic systems.

Interfacial microfluidic transport on micropatterned superhydrophobic textile

Textile-enabled interfacial microfluidics, utilizing fibrous hydrophilic yarns (e.g., cotton) to guide biological reagent flows, has been extended to various biochemical analyses recently. The restricted capillary-driving mechanism, however, persists as a major challenge for continuous and facilitated biofluidic transport. In this paper, we have first introduced a novel interfacial microfluidic transport principle to drive three-dimensional liquid flows on a micropatterned superhydrophobic textile (MST) platform in a more autonomous and controllable manner. Specifically, the MST system utilizes the surface tension-induced Laplace pressure to facilitate the liquid motion along the hydrophilic yarn, in addition to the capillarity present in the fibrous structure. The fabrication of MST is simply accomplished by stitching hydrophilic cotton yarn into a superhydrophobic fabric substrate (contact angle 140 ± 3°), from which well-controlled wetting patterns are established for interfacial microfluidic operations. The geometric configurations of the stitched micropatterns, e.g., the lengths and diameters of the yarn and bundled arrangement, can all influence the transport process, which is investigated both experimentally and theoretically. Two operation modes, discrete and continuous transport, are also presented in detail. In addition, the gravitational effect as well as the droplet removal process have been also considered and quantitatively analysed during the transport process. As a demonstration, an MST design has been implemented on an artificial skin surface to collect and remove sweat in a highly efficient and facilitated means. The results have illustrated that the novel interfacial transport on the textile platform can be potentially extended to a variety of biofluidic collection and removal applications.